1. Field of the Invention
The present invention relates to the structure of an impeller. More specifically, the present invention relates to an impeller for use in a blower.
2. Description of the Related Art
In conventional blowers (blower fans), a motor is arranged inside a cylindrical impeller cup to rotate an impeller with a plurality of blades. The motor includes a stator portion and a rotor portion supported so as to be rotatable with respect to the stator portion. A cylindrical rotor holder is press-fitted to an inner circumferential surface of the impeller cup, so that the impeller is fixed to the rotor holder.
When the rotor holder is press-fitted to the entire inner circumferential surface of the impeller cup, a significantly uneven shape of the rotor holder or the impeller cup leads to an excessive pressure being applied to a portion of the impeller cup, which will result in a breakage of the impeller cup.
JP-A 2008-69672 describes a technique used to overcome the above problem. According to the technique described in JP-A 2008-69672, a plurality of axially extending ribs are arranged in a circumferential direction on the inner circumferential surface of the impeller cup, and the rotor holder is press-fitted to an inside of the impeller cup while also being pressed against a top portion of each rib, so that the impeller is fixed to the rotor holder. According to this technique, when the rotor holder is press-fitted to the impeller cup, the aforementioned excessive pressure due to the uneven shape of the rotor holder or the impeller cup would be absorbed by elastic deformation of the ribs to prevent the breakage of the impeller cup.
The number of blades of the impeller is determined based on the purpose or intended use of the blower or the like. For purposes of cooling an electronic device, such as a server, which is densely packed with components, for example, the blower is required to be capable of providing high static pressure. For the purposes of providing high static pressure, blowers (cooling fans) having an impeller with a small number (e.g., three to five) of blades are suitable. Thus, fans having an impeller with a small number of blades are frequently used in accordance with such demand.
FIGS. 1A, 1B, and 1C are perspective views each illustrating the structure of an impeller with a plurality of blades 102 attached to an outer circumferential surface of an impeller cup 101. In FIG. 1A, the impeller has seven blades 102. In FIG. 1B, the impeller has five blades 102. In FIG. 1C, the impeller has three blades 102. As is apparent from FIGS. 1A to 1C, as the number of blades 102 decreases, the inclination of each blade 102 with respect to an axis of the impeller cup 101 becomes greater, and the length of a root portion 102a of each blade 102 at which the blade 102 comes in contact with the outer circumferential surface of the impeller cup 101 becomes greater.
In general, the impeller cup 101 and the blades 102 are integrally molded in one piece of a resin or the like. In this case, a stress is applied to an outer wall portion of the impeller cup 101 at the root portion 102a of each blade 102, at which the blade 102 comes in contact with the outer circumferential surface of the impeller cup 101. These stresses have certain distributions in circumferential and axial directions with respect to the outer wall portion of the impeller cup 101 depending on the arrangement of the root portions 102a of the blades 102. In the case where the number of blades 102 is large, the stress distributions are substantially even, whereas in the case where the number of blades 102 is small, the stress distributions are uneven. Therefore, when the number of blades 102 is small, the impeller cup 101 tends to undergo a deformation easily. The deformation of the impeller cup 101 will result in reduced adhesion between the impeller cup 101 and the rotor holder, which may lead to the impeller coming off the rotor holder. Moreover, in the case where there is only a small gap between the blades 102 and a case of the blower, the blades 102 may come in touch with the case during rotation of the impeller.
Furthermore, as the number of blades 102 decreases, the weight of each blade increases, and therefore the stress applied to the outer wall portion of the impeller cup 101 becomes greater. Thus, in the case where the number of blades 102 is small, the rotating impeller may undergo a deformation due to the stress, so that the impeller may come off the rotor holder or that the impeller cup 101 may be broken due to the stress. This problem becomes evident when the impeller is caused to rotate at a high speed in order to increase the air flow quantity of the blower.
It is conceivable to increase the wall thickness of the impeller cup 101 in order to overcome the above problem. However, it is difficult to simply increase the wall thickness thereof because of a deformation accompanying contraction when the impeller is molded of the resin, a constraint in terms of the outer diameter of the impeller cup 101, and so on.
It is also conceivable to increase the number of ribs provided on an inner circumferential surface of the impeller cup 101 to enhance the adhesion between the impeller cup 101 and the rotor holder. However, an increase in the number of ribs results in increased resistance when the rotor holder is press-fitted to the impeller cup, which may also lead to the breakage of the impeller cup 101.